1. Field of the Invention
The present invention relates to a toner suited to develop electrostatic images in image forming processes such as electrophotographic, electrostatic recording, and electrostatic printing processes, and a method for producing the toner.
2. Description of the Related Art
In conventional electrophotographic processes, electrostatic latent images are typically formed on photoconductors by various ways using photoconductive materials in photoconductive layers. The latent images are developed using toners, the toner images are optionally transferred onto recording media such as papers, and the toner images are fixed by means of heating, pressing, heating-pressing, or solvent vapor, thereby to produce copies (see U.S. Pat. No. 2,297,691, Japanese Patent Application Publication (JP-B) Nos. 42-23910 and 43-24748).
The toners are typically constructed from toner base particles that contain a binder resin, a colorant, and a charge control agent, and also an external additive such as silica that is deposited on the surface of the toner base particles in order to improve flowability etc.
The toners may be produced by a milling process, in which toner ingredients such as a binder, a colorant, and a charge control agent are mixed and kneaded, followed by cooling, milling and classifying to prepare base particles, then an external additive is mixed and deposited on the surface of the toner base particles.
The toners may also be produced by a milling process, in which a small amount of external additive is included into the toner ingredients before the milling and the classifying steps, then the milling and the classifying steps are carried out.
The methods to add the external additive in the milling step are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 2005-326840, 2005-326841, 2005-326842 and 2006-126587. However, the external additive tends to be embedded into the surface of toner particles in the milling step, therefore, a relatively large amount of the external additive is typically required for assuring its effect, but the relatively large amount of external additive may sometimes cause undesirable phenomena due to the relatively large amount of external additive migrating from the surface at the developing step.
The external additive may also be added before the classifying step, as disclosed in JP-A Nos. 53-58244, 07-104511 and 08-248678. These methods may take an advantage of appropriate classifying ability by virtue of improved toner-flow ability at the classifying step. The component having particle diameters (course particles or fine particles) out of a pre-determined range is recycled into a mixing-kneading step in general for the purpose of resource saving.
When these steps are combined such that an external additive is added before the classifying step and the fine particle component out of the pre-determined range at the classifying step is recycled into the mixing-kneading step; however, there arises such a problem that mixing-kneading torque increases undesirably and the milling ability comes to inadequate at the milling step, since the external additive exists within the recycled toner and the external additive is included in the mixing-kneading step. There may also arise such a problem that the external additive to improve flowability tends to migrate from the surface of toner, deposit on the surface of photoconductors during long-term use, which often acing as nuclei for toner filming.
In toner milling-production methods in which an external additive is added during a classifying step and recycling a particle component out of a pre-determined diameter range into a kneading step, there typically arises such a problem that the filler effect of toner component is likely to be excessive, which deteriorates or increases torque load in the kneading step, and also filming tends to generate on photoconductors.